1. Field of the Invention
The present invention is related to the monitoring technology for Passive Optical Network (PON), especially to a method for monitoring the status of optical links through the TDM (Time Division Multiplexing) technology to get the responses sequentially from the Optical Network Units (ONU).
2. Description of the Prior Art
With the features of big capacity, high reliability and long transmission distance, the optical fiber becomes the backbone of the telephony communication links. The service transmitting the data to clients directly by the optical fiber will be more widespread. Typical services include, for example, fiber to the cabinet (FTTCab), Fiber To The Curb (FTTC), Fiber To The Building (FTTB) and Fiber To The Home (FTTH) which are services generally referred to as FTTx. Based on the structure with no power sources, Passive Optical Network (PON) has the great chance to prevail over other optical linking technology. FIG. 1 shows the structure of a traditional passive optical network 100, in which the Optical Line Terminal (OLT) 110 connects to the major optical link of the optical network 100, and the light downstream optical signals originated from the OLT 100 are broadcasted to the terminal clients, i.e., the Optical Network Units (ONUs) 131-134 through the passive Splitter (SP) 120. The upstream signals from ONUs 131-134 are coupled by the SP 120 and sent to the OLT 110.
With more and more application of optical network, the fault diagnosis becomes important. FIG. 2 shows an optical network 100 with a broken link. Suppose the optical link lying between the SP 120 and the ONU 134 is broken, the service people, however, are generally unable to determine immediately that the fault point is between the OLT 110 and the SP 120 or between the SP 120 and the ONU 134. Besides, the transmission quality might get deteriorated because the optical cable is affected by the environment, such as compressed or impacted under construction, or bent by gravity. Such deterioration process is usually too slow to become aware instantly and will cause loss of business reputation if it is not dealt with properly. Conventionally, an optical link fault detection apparatus, such as the optical time-domain reflectometry (OTDR), may be used to locate the faulty point. The OTDR locates a faulty point or a broken point by comparing the reflecting optical signal pattern with a typical normal reflecting optical pattern previously recorded. With the tree-structure passive network as shown in FIG. 1 and FIG. 2, it is difficult for the OTDR to determine the status thereof, since it is hard to distinguish the faults lying in the branches of the tree.
In view of the foregoing, there is a need to provide an improved optical link monitoring method and system for the optical network to overcome the shortcoming of the prior art, like the OTDR. It can monitor and record the transmission quality of the optical network to replace the deteriorated optical link in time, and locate the fault immediately to shorten the recovering time.